Touch probe for CMM including digital signal communication

Abstract

A compact CMM touch probe may include internal digital processing, a regulated power circuit, a trigger signal generating circuit, an interface connector limited to two electrical connections, and a differential signal configuration that inputs and outputs differential digital communication signals (including touch trigger signals) through the two connections, superimposed on a CMM supply voltage connected to them from the CMM. A specialized supply isolation circuit is configured in combination with various probe components and operating parameters, to couple the regulated power supply circuit to the CMM supply voltage at the two connections, while isolating it from loading the differential digital signals, which are AC coupled to the two connections. The differential digital signals may normally comprise touch trigger signals, and intermittent control and data signals, during separate respective time periods. In various embodiments, at least a majority of the differential digital signals are formatted using a DC-balanced encoding scheme.

What is claimed is: 
     
       1. A compact touch probe for a coordinate measuring machine (CMM), including internal digital signal processing,
 the compact touch probe comprising:
 a displacement sensor configured to output at least one sensor signal that is responsive to displacement of a stylus attached to the compact touch probe, 
 an interface connector including first and second electrical connections that electrically connect the compact touch probe to the CMM; 
 touch probe electronic circuitry, included entirely within a housing of the compact touch probe, comprising:
 a regulated power supply circuit; 
 probe operating circuits and memory; 
 a trigger signal generating circuit that inputs the at least one sensor signal and generates a touch trigger signal corresponding to a displacement of the stylus; and 
 a digital communication circuit including a differential signal configuration that inputs and outputs differential digital signals through the first and second electrical connections, 
 
 
 wherein: 
 the touch probe electronic circuitry further comprises a supply isolation circuit configuration that is coupled to the first and second electrical connections to receive a connected supply voltage from the CMM through the first and second electrical connections and that is configured to isolate the regulated power supply circuit from loading the differential digital signals that pass through the first and second electrical connections, the supply isolation circuit configuration comprising:
 a first end of a first winding of a differential mode choke coupled to the first electrical connection, with a second end of the first winding of the differential mode choke connected to a positive input of the regulated power supply circuit, 
 a first end of a second winding of the differential mode choke coupled to the second electrical connection, with a second end of the second winding of the differential mode choke connected to a negative input of the regulated power supply circuit, and 
 at least a first supply isolation capacitor coupled between the positive and negative inputs of the regulated power supply circuit, 
 
 the digital communication circuit comprises the differential signal configuration that is AC coupled to the first and second electrical connections through a communication isolation configuration comprising a first communication isolation capacitor coupled in series to the first electrical connection and a second communication isolation capacitor coupled in series to the second electrical connection, and the differential signal configuration is configured to input and output the differential digital signals through the first and second electrical connections, the differential digital signals comprising:
 control and data signals that are input and output superimposed on the connected supply voltage during control and data time periods, and 
 touch trigger signals generated by the trigger signal generating circuit that are output superimposed on the connected supply voltage during trigger signal time periods that are different than the control and data time periods; and 
 at least a majority of the differential digital signals are formatted based on a serial communication code scheme that is nominally DC-balanced. 
 
 
     
     
       2. The compact touch probe of  claim 1 , wherein the interface connector is a combined mechanical mounting and electrical interface connector for the compact touch probe. 
     
     
       3. The compact touch probe of  claim 1 , wherein the first and second electrical connections are the only electrical connections in the interface connector. 
     
     
       4. The compact touch probe of  claim 1 , wherein all of the control and data signals are formatted based on the serial communication code scheme that is nominally DC-balanced. 
     
     
       5. The compact touch probe of  claim 1 , wherein the touch trigger signals are included in a normal serial communication protocol that is automatically repeated at a probe sample cycle rate during ongoing touch probe measurement operations. 
     
     
       6. The compact touch probe of  claim 5 , wherein when the stylus is not contacting a workpiece such that there is no stylus displacement, the trigger signal generating circuit generates a null touch trigger signal that is included in the normal serial communication protocol. 
     
     
       7. The compact touch probe of  claim 5 , wherein the probe sample cycle rate is at least 50,000 samples per second. 
     
     
       8. The compact touch probe of  claim 5 , wherein the normal serial communication protocol is formatted or encoded such that it is nominally DC-balanced. 
     
     
       9. The compact touch probe of  claim 1 , wherein the negative input of the regulated power supply circuit is a circuit ground connection. 
     
     
       10. The compact touch probe of  claim 1 , wherein the communication isolation configuration comprises the first communication isolation capacitor coupled in series directly from the first electrical connection to a first transceiver node of the differential signal configuration, and the second communication isolation capacitor coupled in series directly from the second electrical connection to a second transceiver node of the differential signal configuration. 
     
     
       11. The compact touch probe of  claim 1 , wherein the communication isolation configuration comprises:
 the first communication isolation capacitor coupled in series to the first electrical connection and to a first end of a first winding of a common mode choke, with a second end of the first winding of the common mode choke coupled to a first transceiver node of the differential signal configuration; and 
 the second communication isolation capacitor coupled in series to the second electrical connection and to a first end of a second winding of the common mode choke, with a second end of the second winding of the common mode choke coupled to a second transceiver node of the differential signal configuration. 
 
     
     
       12. The compact touch probe of  claim 1 , wherein the differential mode choke has an impedance on at most 25 μH for frequencies associated with the differential digital signals. 
     
     
       13. The compact touch probe of  claim 12 , wherein the compact touch probe comprises a cylindrical housing and has a diameter of at most 15 mm. 
     
     
       14. The compact touch probe of  claim 1 , wherein the majority of the differential digital signals have a bit rate of at most 3 Mbps. 
     
     
       15. The compact touch probe of  claim 14 , wherein the majority of the differential digital signals have a bit rate of at most 1.5 Mbps. 
     
     
       16. The compact touch probe of  claim 1 , wherein at least the digital signal configuration is coupled to an unregulated voltage at the positive input of the regulated power supply circuit, such that it is powered by that unregulated voltage.